A common approach for obtaining a product that combines desirable properties of different polymers is to mix individual homopolymers and achieve a blend exhibiting the desired combination of functions. Unfortunately, thermodynamic incompatibility of homopolymers often causes interfacial instability and phase separation of polymer blends resulting from the reduced van der Waals or other inter-macromolecular interactions. An obvious strategy to overcome these obstacles is to covalently attach immiscible homopolymer segments to one another to form a stable block copolymer backbone that maintains the properties of each homopolymer segment.
For almost two decades, controlled radical polymerization (CRP) has dominated the field of polymer synthesis. Due to pseudo-living features, CRP methods including nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP), and reversible addition-fragmentation chain transfer (RAFT) polymerization have facilitated thermodynamically controlled conditions for the synthesis of well-defined block copolymers with a narrow dispersity. These advances have enabled many applications, ranging from drug delivery systems to diagnostic imaging processes, new separation membranes, and others. While CRP methods have brought improvement to the field, problems still exist. Drawbacks of CRP methods include time-consuming synthesis, multiple purification steps, and molecular weight limitations. For instance, existing CRP block copolymer synthetic methods can produce a maximum block copolymer molecular weight of about 200,000 and the process takes several days or longer.
Emulsion polymerization is a copolymer formation approach that is well suited for the synthesis of ultrahigh molecular weight polymers. Unfortunately, limited copolymer composition control and high dispersity have presented significant drawbacks. Due to fast kinetics of initiation, propagation, and termination steps, copolymer structural control during this statistical polymerization is difficult to achieve.
What are needed in the art are methods for forming high molecular weight block copolymers. One-step methods that can be carried out quickly and produce ultrahigh molecular weight block copolymers with high reproduction consistency would be of great benefit in the art.